added support for local decs, plus pretty printing

[brandonspark]

• Problem: Currently, there is no support for local declarations. Consider the following program:

local
  val x = 2
in
  val y = x
end

This will cause the following error when parsed:

λ ~/P/parse-sml on main ◦ cat test.sml                    18:58:12
local
  val x = 2
in
  val y = x
end
λ ~/P/parse-sml on main ◦ ./main test.sml                 18:58:14
local
  val x = 2
in
  val y = x
end

Lexing succeeded.
Parsing...

-- ERROR: NOT YET IMPLEMENTED --------------------------- test.sml

Unexpected token.

1| local
   ^^^^^
(TODO: Sam: see Parser.parse.consume_oneDec)

• Solution: This PR adds in support for local declarations, along with pretty printing for local declarations in the AST. There's also some tests.

Then, we correctly parse as:

λ ~/P/parse-sml on add-local-dec  ./main test.sml         18:59:45
local
  val x = 2
in
  val y = x
end

Lexing succeeded.
Parsing...

local val x = 2 in val y = x end

Parsing succeeded.

[shwestrick]

Nice work!! (On this one and the other PRs too!)